What Is A Genetic Disorder and How Does It Occur?

Many prenatal tests look for genetic disorders -- but what exactly is a genetic disorder, and what can you do (if anything) to prevent one from happening? Find out, and get familiar with the most common genetic disorders.

Genes dictate just about everything about our bodies, from the color of our eyes to the shape of our ear lobes. But sometimes they get damaged or mixed up in ways that cause illnesses from birth. Is it because of what mothers eat, drink, or other conditions in the womb? No.

"Most of them are just random errors that accumulate over time as the DNA divides," explains Ellen Simpson, Ph.D., a genetics counselor in the prenatal diagnostic center at the University of California, San Francisco Medical Center. "Every time our cells divide, they have to make a copy of our DNA and these mistakes accumulate. Most genetic disorders are due to ancient mutations that first happened a long time ago."

Many of us are walking around with between 5 and 10 recessive mutations, meaning we don't get sick because we only have one copy of the mutated gene and are just carriers. But, if we make a baby with someone who is also a carrier of that mutation, there's a 25 percent chance the child will get the mutation from both of you and officially have the disorder.

That's the case in illnesses like cystic fibrosis, sickle cell anemia, and Tay-Sachs. There are other genetic disorders, such as Marfan syndrome, achondroplasia, and Huntington Disease, in which a child only needs to inherit the genetic mutation from one parent.

Chromosomal disorders, like Down syndrome, aren't inherited through genes -- but they happen when the baby receives either too many or too few copies of a gene or chromosome.

Parents who know they carry a gene mutation, or are concerned they might, can have preconception genetic testing to determine their future children's risk for certain disorders. Pregnant women routinely have several different screening tests to determine whether their child is likely to have a genetic disorder.

Most Common Genetic Disorders

Cystic Fibrosis

Cystic fibrosis is a life-threatening condition that causes lung damage and digestive problems. About 30,000 people in the U.S. have the disease, which mostly affects Caucasians of Northern European descent. Organizations like the National Institutes of Health, the American College of Obstetricians and Gynecologists, and the American College of Medical Genetics have recommended nationwide genetic testing for CF for all adults since 2002. According to a recent study in the New England Journal of Medicine, the number of babies born with cystic fibrosis in Massachusetts dropped by about 50 percent in the four years after this recommendation was made and testing came into wider use compared to the four years before.

Sickle Cell Disease

This genetic condition is most common in people of African and Mediterranean backgrounds, causes a blood disorder that leads to anemia, a weakened immune system, and other health complications.

Thalassemia

Thalassemia is another blood disorder common to people of African and Mediterranean descent. It typically causes anemia and bone growth and liver problems; in severe cases, some babies born with the condition may not survive.

Tay-Sachs Disease,

This disorder of the central nervous system which is usually fatal in early childhood mainly affects people of French Canadian and Eastern European Jewish descent, Eastern European Jews also face an increased risk for another nervous system disorder called Canavan disease as well as a number of other conditions including familial dysautonomia, familial hyperinsulinism, and Gaucher disease. Your doctor can screen for all of these conditions at the same time.

Fragile X Syndrome

This condition that can cause developmental problems, including learning disabilities and mental retardation, is not linked to a specific ethnic background. Reviewing your family's health history with a doctor or genetic counselor may help you decide whether you should be screened for Fragile X.